BMS not shutting down when sense wires are disconnected.

[kingjamez]

I have been happily using my SUPower BMS: http://www.ebay.com/itm/72V-74V-84V-20S-60A-20x-3-6V-3-7V-4-2V-Lithium-ion-Li-ion-Li-Po-Battery-PCB-BMS-/321745442492?hash=item4ae985d6bc:g:BeUAAOSwv0tVRyqE

I can charge and discharge through it just fine. It is wired exactly as shown in the instructions you can find on the ebay page. I haven't tested it's protection capabilities until today. While discharging with a load (light bulb) I disconnected one of the balance connectors for one pack of 4 cells (of the 20 cell pack). The BMS didn't shut down. No voltage at all for 4 cells and it still kept discharging.

Any ideas on what could be wrong? Does it need to sense some voltage (ie. 2.9, 2.8 etc.) in order to trigger the shutdown?

-Jim

 

[fechter]

Yes, it needs some voltage to trigger the circuit. Try a 1.5v alkaline battery.
This is typical of most inexpensive BMS circuits. They don't detect an open sense wire.

 

[kingjamez]

Good idea on the alkaline battery test. That worked. I hooked up a C battery between two adjacent sense wires and the BMS immediately turned off. After reconnecting the real battery back in, the BMS turned the high voltage back on. Perfect!

Thanks!

-Jim

 

[amberwolf]

Yeah, this is what causes damage to some (probably a lot, actually) of packs, where a balance wire is broken off or never even connected at the factory--the disconnected ones may run well down below their LVC repeatedly, or well above their HVC during charge, and get worse and worse over time until a cell fails.

It's even possible for this to cause a fire, and has been suspected in a couple of the ones posted around ES but cant' really be proven.

 

[Punx0r]

It appears to be a huge oversight with many BMSs

 

[fechter]

More advanced versions will detect an open or shorted balance wire and trip, but the more common varieties depend on power from the cell to send the signal to shut down.

 

[kingjamez]

After some more testing, the BMS works fine for the 95% case. It's clear though that it's not a hands-off tool like the more advanced ones (like the one in my Leaf). One way to mitigate the risk would be to not use connectors and solder the balance wires directly to the BMS. I like the flexibility of having connectors on the balance wires, so I'll just manually check the voltage at the BMS regularly. If a wire comes loose, it isn't the end of the world, especially if you don't discharge near empty very often. I'd say weekly checks of the connections is probably very sufficient.

-Jim